The wave length of the light produced by a laser depends upon the nature of the materials utilized by the laser to produce its radiation. For example, Helium-Neon lasers produce a single wavelength near the lower end of the visible spectrum. An Argon laser is capable of producing a number of wavelengths, most of which are in the visible portion of the spectrum. By contrast, carbon dioxide lasers (i.e. lasers which utilize a mixture of gases including carbon dioxide, and in which vibratory changes in the carbon dioxide molecule are responsible for the generation of the radiation) produce an emission having a wavelength longer than the wavelengths of the visible spectrum. Hence, the radiation from a carbon dioxide laser is in the infra-red region, and is invisible to the eye.
Recent developments in laser technology have allowed the construction of carbon dioxide lasers capable of a power output in the range of 20,000 kilowatts and up. A typical construction produces a beam (prior to focusing) which may have a diameter of 2 inches to 4 inches. It is important to be able to measure the actual power output in such a beam relatively accurately.
The accurate measurement of C.W. laser power of a few hundred watts, up to about 1 kw, is today relatively straight-forward, since several optical power meters are now available commercially in this power range. However, similar measurements at the multi-kilowatt to tens of kilowatt level are still complicated by the fact that no power meters for this power range can yet be purchased. The literature has described several approaches to the construction of optical power meters suitable for this purpose (1-5); but most have been far less convenient to use than the corresponding lower power commercial units. In an attempt to overcome this deficiency a new type of high level optical power meter has been developed which is not only simple and inexpensive to build, but in addition, does not require calibration.
Although several different methods have been proposed and attempted in the prior art, experience has shown that in the design of power meters, for any wavelength range, it is highly desirable to utilize a calorimeter type geometry so that the difficult problems of initial calibration, and subsequent degradation, can be avoided. In the microwave portion of the EM spectrum such techniques have long been commonplace, since the early development of precision water loads for rectangular and circular waveguide. There a flowing water cone or wedge was utilized to provide a near reflectionless impedance match to the radiation source; thereby providing a convenient and absolute method for measurement of high average EM power (6).
A somewhat similar approach was used in the design of a "circulating liquid" calorimeter for the detection of high power pulsed laser signals (7). The absorbing liquid wedge structure of reference (7) is however, not appropriate to many types of lasers, including CO.sub.2 lasers, because the walls of the liquid containment vessel are not compatible with the emission spectrum. Although in principle one can envisage a liquid containment vessel made from an IR transmitting material such as ZnSe, the concept is still not practical for a high level power meter. Not only would a ZnSe enclosed water wedge be prohibitively expensive, but experiments have shown that the presence of containment vessel walls in a high average power CO.sub.2 laser calorimeter introduce inconsistency and error into the measurements. Moreover, the use of a containment surface of any kind severely restricts the power handling capacity of the device.